The game stars an unnamed marine, a survivor of Bravo Team in June 2145 (several months before Doom 3 (November 15, 2145). He awakens, and is soon confronted by Dr. Garret, who presents him Sam, a flying droid capable of opening doors and hacking computers. Together with Sam, the survivor makes his way through the Mars facility, battling zombies and demons on his way through the base. The task is to reach a port, where a spaceship full of surviving members of UAC facility is preparing to launch off. On his way, marine meets few survivors (who dies in front of his eyes), collects valuable information through Sam, and visits Hell, where he closes the demon-spawning portals. Eventually, marine is forced to leave Dr. Garret behind, and Sam sacrifices itself to support a closing door to the spaceship, leaving only the data implicating UAC behind. The marine successfully boards the ship and leaves Mars with a few other survivors.